The present invention relates to video tape recording and more particularly to a system for extending the playing time of standardized video cassettes in which each of two rotating video heads, displaced relative to each other by 180.degree., records or scans one slant track per television field.
In the standard video cassette intended for home use, a magnetic tape located in a cassette is utilized. When the cassette is positioned in the video equipment, the tape is pulled from the cassette and positioned for operation. In order to permit interchangeability of the cassette (i.e., in order to insure that a cassette recorded on one machine can be played back on another machine) certain system standards have been established which are adhered to by the industry. These standards include the geometry of the slant tracks, the speed of rotation of the head drum, the number of video heads mounted on the drum, and the transport speed of the tape past the head drum. In one popular cassette system two video heads are mounted on the head drum 180.degree. apart from each other and displaced to record or scan along one slant track. With 50 fields per second (as is common in most European countries) the head drum must therefore rotate at 1500 RPM. In order that the common line interlace of 312.5 lines per field is recorded and reproduced correctly, a tape speed must be chosen so that the spacing between the tracks (in the lengthwise direction of the tape) corresponds to an odd multiple of half lines. In the aforementioned system, this multiple was chosen as 11.
As a result of the above established standards, the maximum playing time of a cassette is limited to about an hour utilizing the thinnest tapes available. This playing time is not sufficient for certain forms of desired programs (e.g., such as full-length motion pictures). Thus, a considerable need exists for a system to extend the playing time of a standard cassette while maintaining provision for normal recording and playback capabilities.
It has heretofore been proposed (in German Pat. No. 1,437,141 for example) that the playing time of a given length of tape could be extended through the slant track method by recording only the even or odd television fields but scanning them twice during playback with the tape transport moving at roughly half the normal speed. In practice, a head drum which rotates at 1500 RPM and carries two video heads is utilized. The magnetic tape is looped 180.degree. over the head drum. For recording, only one of the heads operates while for playing back both heads scan the same track. It is also known that in this case the two heads must no longer be precisely 180.degree. apart from each other but the additional head position must be displaced by a slight amount from 180.degree.. In practice this displacement distance is on the order of a millimeter and thus, it would not be practical to mount an additional head displaced by this slight amount.
The principal object of the present invention is to provide an improved video cassette machine capable of functioning in both a standard normal mode and an extended play mode.
A further object is to provide such a machine which can be readily switched from one mode to the other without requiring any intervention with the drive mechanism of the head drum.